We investigate the theoretical origin of hydrogen bonding, whether it is primarily due to classical electrostatic (ionic or dipole-dipole force) as advocated by most of the chemistry textbooks, or quantum covalency (resonance-type charge–transfer delocalization). We employ density functional theory, B3LYP with aug-cc-pVTZ basis set along with natural bond orbital (NBO) methods and techniques to investigate several representative binary H-bonding complexes. For each complex, we evaluate a variety of measurable experimental descriptors, which are considered a signature of hydrogen bonding, such as ΔνHA (IR frequency shift of hydride base), ΔRB..H (H-bond penetration distance), ΔRH―A (covalent bond length shift). In addition, we provide theoretical descriptors that are related to intermolecular resonance covalency (B:….HA →BH+….A– ) such as QCT and donor-acceptor interaction energy (ΔE(2)).

A project for students in an independent study class is described in which they explore applications of modern electronic structure methods and natural bond orbital methods to investigate the origin of hydrogen bonding for a variety of binary complexes. They perform computational investigation using density functional theory, B3LYP with aug-cc-pVTZ basis set to optimized each binary complex, followed by natural bond orbital analysis, which provides best single “natural Lewis structure” (NLS) representation of chosen wavefunction (Ψ). Our conclusion from this work finds charge transfer from a donor Lewis-type NBO (nB) to an acceptor non-Lewis-type NBO (σHA*) is the primary cause for H-bonding. We provide variety of experimental and theoretical descriptors to support the conclusion, such as IR frequency shift (ΔνHA), H-bond penetration distance (ΔRB..H), and donor-acceptor stabilization energy (ΔE(2)).